Directional boring apparatus for making holes through soil are well known. The directional borer generally includes a series of drill rods joined end to end by joint couplings to form a drill string. The drill string is pushed or pulled though the soil by means of a powerful hydraulic device such as a hydraulic cylinder or gear rack mounted on the HDD machine. A drill head for boring in soil, rock or both is attached to the end of the drill string with a joint coupling and may include an ejection nozzle for water or other drilling fluid to assist in boring. In other applications, tools such as reamers, pipe bursters, impact machines, slitters and pullers are attached to the end of a drill string with a joint coupling and are used to place underground pipelines.
During forward drilling, the joint coupling trails the leading bit where cutting is performed. Generally, the joint is exposed to limited amounts of abrasion as the joint is typically the same size as the sonde housing and smaller than the 3 to 8 inch diameter bore created by the bit. Even during these conditions, joint couplings between a drill string and tool are subjected to severe torque loadings and longitudinal stresses in these operations.
Back reaming exposes the tooling to additional wear, especially the joint coupling. Additionally, the cuttings or local soils often collapse on the first drill rod and the joint coupling, increasing the abrasion they sustain during the ream and shortening their useful life overall. Drilling fluid reduces the propensity for this to happen by permitting the abrasive soil to flow away. In existing designs the reamer is designed to discharge fluid at multiple strategic locations to aid the back reamer in cutting rock or soil. This makes the reaming and cutting process easier, but does not provide the joint couplings or drill string with any protective drilling fluid.
Additionally, during the course of any drilling operation, be it forward drilling or back reaming, significant and unavoidable wear occurs on the threading between the male and female ends of the joint coupling, drill string and tooling. This is a serious problem, because when the threads no longer hold the couplings securely, the worn parts must be replaced. An existing design, U.S. Pat. No. 6,860,514 addresses this issue by providing re usable threading, but does not address the fundamental vulnerability of the sleeve encasing the joint threadings to abrasion and damage.
Failure of a joint in a horizontal drilling or back reaming operation can result in not only a tool stuck in a borehole or pipe, necessitating costly and time consuming excavation to recover the tool, or form a bore around the location at which the tool was lost, but can prevent separation of the reamer from the drill string in extreme cases of abrasion where the starter rod is worn through to the point of fracture by the abrasive conditions. This necessitates costly in ground repairs and part replacements. It is clear that a means for extending the lifetime of the joint coupling is needed. The present invention addresses this need.